Electroconductive zinc oxide is widely made practicable as an additive for imparting electroconductivity to paint, rubber, resin or the like. In the field of paint out of these, the paint workability thereof generally lowers when the viscosity of the paint becomes high; therefore, powder having a relatively large particle size is used in many cases so as not to make the paint viscosity high. On the other hand, in the field of rubber, resin or the like, the viscosity of the composition thereof is hardly worth consideration; therefore, electroconductive zinc oxide powder having a relatively small particle size is used.
And now, several processes have already been suggested as the process for producing an electroconductive zinc oxide powder. For example, JP-A-1-126228 discloses a process for stirring three components of zinc oxide, a water-soluble or water-dispersible aluminum compound, and ammonium carbonate or the like in an aqueous dispersion system in the presence of inorganic fine powder, filtrating and dehydrating the resultant, and then heating the dehydrated substance at a temperature of about 600° C. or lower in a non-oxidizing atmosphere, thereby producing an electroconductive zinc oxide fine powder which has an average particle size of approximately 0.1 μm and is excellent in transparency.
However, about conventional electroconductive zinc oxide powders, examples of which include one based on the above-mentioned process, the average primary particle size thereof is restricted to approximately 0.1 μm even if the particle size is made small. According to the particle size at such a level, a sufficient electroconductivity is not easily obtained by the addition of the electroconductive zinc oxide powder alone for the following reason: even if the powder is incorporated into a base material such as rubber or resin and uniformly dispersed therein, the zinc oxide particles are not brought into close contact with each other in the powder-incorporated composition.
If the particle size of electroconductive zinc oxide powder can be made into the form of superfine particles, which has a particle size that is one or more figures smaller than the present particle sizes, contact points between the particles can be increased by dispersing the superfine particles uniformly into a base material such as rubber or resin. As a result, a rubber composition or resin composition having a low resistance value would be obtained.
As another example of electroconductive zinc oxide in such a fine particle form, JP-A-10-236822 discloses a process of heating and aging a liquid mixture of a carboxylic acid zinc salt and an alcohol to produce a zinc oxide precursor, mixing this precursor with a metal hydroxide or a compound which is reactive with water to produce a metal hydroxide, distilling off the alcohol, and then firing the resultant. It is stated that this process makes it possible to yield a zinc oxide fine powder having an average particle size of about 0.001 to 1 μm.
JP-A-7-69631 discloses a process of adding a hexamethylenetetramine solution or a urea solution to a liquid mixture of a zinc salt and an aluminum salt; hydrolyzing the resultant at a pH of 5.5 to 7.5 to produce a flaky basic zinc-based coprecipitate; adding to the flaky coprecipitate a water-soluble compound of at least one element selected from the group consisting of antimony, indium, tin, zirconium and titanium to coat the surface of the flaky zinc coprecipitate; and then firing the resultant. It is stated that the zinc oxide obtained by this process has an average thickness of 0.1 to 2 μm, an average particle size of 1 to 100 μm, and an electric resistance of 1×103 Ω·cm or less.
As a process for producing zinc oxide in the form of fine particles which has a different use purpose but appears to have electroconductivity, JP-A-11-279525 discloses a doped electroconductive zinc oxide powder which (a) comprises at least one element selected from IIIB group elements and IVB group elements, wherein (b) the arithmetic average particle size in a given direction is from 3 to 100 nm in an electron microscopic projection image thereof, and (c) the total content of the IIIB group element(s) and the IVB group element(s) is from 1 to 15% by mole.
However, the inventors' investigation demonstrates that in the case of dispersing the zinc oxide powder produced by a conventional process as described above into a base material such as rubber or resin, the property of the base material can be maintained but a fall in the electric resistance value is insufficient or both of the base material property and the electric resistance value are insufficient in any case. Thus, it cannot be said that the purpose of the addition of the electroconductive zinc oxide powder can be sufficiently exhibited.
Paying attention to such a situation, the inventors have made the present invention. An object thereof is to provide an electroconductive zinc oxide powder having excellent dispersibility and electroconductivity-imparting property such that the incorporation of the powder into a base material such as rubber or resin causes a composition having a largely-decreased electric resistance value to be yielded without lowering the property of the base material; a process for the production thereof; and further an electroconductive composition wherein characteristics of the electroconductive zinc oxide powder are utilized to reform rubber or resin.